Original paper

Ductile and brittle deformations in the Sopron Gneiss Formation (Sopron Mts., W-Hungary) and their implication on the formation of the Pannonian Basin

Benkó, Zsolt; Bugledits, Éva; Récsi, András

Abstract

The crystalline complex of the Sopron Mts. at the eastern end of the Austroalpine nappe system and on the western rim of the Pannonian Basin is key area to investigate the relationship of orogenic and extensional-rift structures that led to the formation of the Pannonian Basin.Field analysis of brittle and ductile deformation zones of the gneissic rocks combined with structural data from the broader vicinity of the Sopron Mts. supported us to delineate the structural evolution of the Sopron Mts. Seven structural events have been distinguished. S and SW dipping S2 foliation along with segregation quartz veins (D1 phase) formed during the Late Cretaceous peak metamorphism. Thick milky quartz veins crosscutting S2 foliation planes (D2 phase) formed during or subsequently the peak metamorphism and were deformed by S2 foliation parallel thrust planes under ductile-brittle conditions (D3 phase). Rapid uplift of the metamorphic complex still during the Late Cretaceous reactivated the low-angle thrust faults as normal faults. Displacement along majority of the thrust and reactivated thrust planes was only small scale due to the relative dry conditions during metamorphism. Large scale displacements were confined on some major shear zones characterized by mylonite and leucophyllite. Reactivation of thrust faults led to disintegration and block rotation of the different units of the metamorphic complex.During the Paleogene the complex emerged to the brittle upper part of the crust and exhumed during the Late Palaeogene-Early Miocene. Flower structures, dextral Riedel fault sets and conjugated Mohr faults indicate N-S compression and perpendicular E-W extension (D4 phase) that could be connected to the onset of the eastward extrusion of the Alcapa Unit from the Alpine collision zone during the Late Palaeogene. SE dipping steep major normal faults with thick cataclasite zones (D5 phase) are interpreted as upper termination of Early Miocene low-angle large scale detachment faults related to the synrift gravitational collapse of the Alpine-Intra Carpathian realm. ENE-WSW oblique sinistral and NE-SW dextral faults formed in a stress field characterized by NNW-SSE extension and perpendicular ENE-WSW compression during the late-Miocene (D6 phase). Postrift thermal subsidence of the Pannonian Basin during the late Miocene-Pliocene resulted in formation of NW dipping normal faults and reactivation of elder SE dipping normal faults (D7 phase).However, it is generally accepted that as low-angle normal faults reactivated Cretaceous thrust faults were major detachment zones during the Early Miocene rift phase of the Pannonian Basin, our work revealed that these faults in the Sopron Mts. were inactive during the Late Miocene due to their uplifted position and improper orientation.

Keywords

eastern-alpspannonian basinsopron mtsgneissmylonitelow-angle thrust faultsfault reactivation